The present invention relates to a magnetic recording medium mounted on a variety of magnetic recording devices such as an external memory device of a computer. The invention also relates to a method for manufacturing such a medium.
Remarkable technology development of magnetic recording devices in recent years is pushing for magnetic recording media with higher coercive force and lower noises. For addressing such demands, various proposals have been made concerning compositions and structure of a magnetic layer and materials of a nonmagnetic underlayer.
Recently, a magnetic layer has been proposed having a structure in which ferromagnetic crystal grains are surrounded by nonmagnetic and nonmetallic substance, such as an oxide. Such a magnetic layer is generally called “a granular magnetic layer”. The granular magnetic layer is considered that interaction between the ferromagnetic crystal grains is decreased because the individual ferromagnetic crystal grain is physically separated by the nonmagnetic and nonmetallic grain boundary phase.
U.S. Pat. No. 5,679,473 discloses that a magnetic recording medium having a granular magnetic layer exhibits high coercive force and low noises. The granular magnetic layer in this reference was formed by RF sputtering using a target of an alloy of CoPt system incorporating an oxide such as SiO2. The reference, U.S. Pat. No. 5,679,473, also discloses the diameter of the crystal grains constituting the granular magnetic layer, the distance between the crystal grains, and macroscopic composition of the granular magnetic layer. The granular magnetic layer as disclosed does not need substrate heating for controlling crystal structure. Consequently, the granular magnetic layer has advantages of superior productivity and possibility of forming on an inexpensive nonmagnetic substrate made of plastic, for example.
In order to obtain a magnetic recording medium with excellent recording performances using the granular magnetic layer, it is necessary to increase coercive force and anisotropic magnetic field, and also necessary to suppress interaction between the crystal grains in the magnetic layer by enhancing magnetic isolation of each crystal grain. Thus, when a granular magnetic layer is deposited using a target containing an oxide such as SiO2 and a ferromagnetic alloy such as cobalt alloy, the oxide and the ferromagnetic alloy composing the target are desired to be well separated to one another in the deposited magnetic film. Consequently, the target and its material must be appropriately selected so that the ferromagnetic crystal grains made of the ferromagnetic alloy and the grain boundary region made of the oxide are well separated after deposition by sputtering.
Various sputtering methods that are conducted in an argon gas atmosphere are known among generally employed deposition techniques. In most sputtering methods, the impact of the argon ions in plasma at the collision with the target readily causes decomposition of the oxide such as SiO2. The silicon atoms liberated by decomposition of SiO2 readily diffuse into the ferromagnetic alloy such as cobalt base alloy and are liable to cause degradation of magnetic characteristics. Thus, the composition of the actually deposited magnetic layer is often different from the composition of the target supplied for sputtering. If the same target is used, but the sputtering is conducted under a different condition, then the composition of the deposited magnetic layer possibly changes.
As described above, even if a suitable target is selected, the target can not necessarily achieve its full ability when the target is deposited as a magnetic layer. In order to adequately control the granular magnetic layer, it is essential to control the composition of the magnetic layer resulted by deposition process. Nevertheless, such a guideline has not elucidated yet.
Accordingly, it would be desirable to provide a magnetic recording medium with excellent magnetic and magnetic conversion characteristics by using a granular magnetic layer having a specified composition, and a method for manufacturing such a medium.